NORTH CHARLESTON, S.C. (AP) — With just over two weeks until voting begins, Republican presidential candidates Donald Trump and Ted Cruz firmly asserted their standing atop the GOP race in a fiery debate, overshadowing a crowded field of rivals still grappling for a way to overtake the front-runners.

Thursday night's debate underscored that the competition between Trump and Cruz will be rough-and-tumble in the days leading up to the Feb. 1 Iowa caucuses, a shift from the relative civility that's defined their relationship until now. The candidates tangled over Cruz's eligibility to serve as commander in chief and the real estate mogul's "New York values," with Trump besting his rival with an emotional recounting of his hometown's response to the Sept. 11 attacks.

"When the World Trade Center came down, I saw something that no place on Earth could have handled more beautifully, more humanely than New York," Trump said. "That was a very insulting statement that Ted made."

Trump renewed his suggestion that Cruz may not be eligible to serve as commander in chief, saying the senator has a "big question mark" hanging over his candidacy, given his birth in Canada to an American mother. Cruz suggested Trump was only turning on him because he's challenging for the lead in Iowa — and the businessman agreed.

Thursday's debate was one of the last high-profile opportunities other candidates on stage had to sway voters' views. But none appeared to emerge with a breakout moment.

Florida Sen. Marco Rubio, who holds a slight advantage over the field of more mainstream candidates, found himself in heated exchanges with both Cruz and New Jersey Gov. Chris Christie.

Rubio likened Christie's policies to President Barack Obama's, particularly on guns, Planned Parenthood and education reform — an attack Christie declared false. Seeking to undermine Rubio's qualifications for president, Christie suggested that senators "talk and talk and talk" while governors such as himself are "held accountable for everything you do."

Cruz confronted Rubio late in the debate over his support for a Senate bill that would have created a pathway to citizenship for people in the U.S. illegally, an unpopular position among GOP primary voters. Rubio tried to flip the criticism around on Cruz, accusing him of switching positions on immigration himself, as well as on numerous other issues.

"That is not consistent conservatism," Rubio said. "That is political calculation."

Cruz was also on the defensive about his failure to disclose on federal election forms some $1 million in loans from Wall Street banks during his 2012 Senate campaign. He said it was little more than a "paperwork error."

Rubio and Christie are among the candidates seeking to break out of the establishment pack, particularly in the New Hampshire primary, which quickly follows the leadoff Iowa caucuses. The race in Iowa has settled into a tight, two-way contest between Trump and Cruz.

Thursday night's debate came at the end of a week that has highlighted anew the deep rifts in the Republican Party. South Carolina Gov. Nikki Haley, a rising GOP star, was widely praised by many party leaders for including a veiled criticism of Trump's angry rhetoric during her response to Obama's State of the Union address — only to be chastised by conservative commentators and others for the exact same comment.

Trump said he wasn't offended by Haley's speech and argued his anger is justified.

"I'm very angry because our country is being run horribly," he said. "And I will gladly accept the mantle of anger."

Trump also stuck with his controversial call for temporarily banning Muslims from the United States because of fear of attacks emanating from abroad. He said he had no regrets about the proposal and noted his poll numbers went up after he announced the plan.

Former Florida Gov. Jeb Bush, who has emerged as a frequent critic of Trump, urged the front-runner to reconsider the policy.

"What kind of signal does that send to the rest of the world?" said Bush, who has struggled to gain any momentum in the race and often appeared overshadowed Thursday night.

Ohio Gov. John Kasich also broke with Trump on the Muslim ban, but like the entire GOP field, called for at least a temporary halt on the Obama administration's plan to allow thousands of Syrian refugees into the country.

"I've been for pausing the Syrian refugees," Kasich said. "But we don't want to put everybody in the same category."

On the economy and national security, the candidates agreed any of them would be better than Obama or Hillary Clinton.

"On Tuesday night, I watched story time with Barack Obama, and it sounds like everything in the world was going amazing," Christie said.

Bush suggested the country was less safe under Obama and declared Clinton would be a "national security disaster."

Rubio went even further, saying Clinton was "disqualified for being commander in chief," accusing her of mishandling classified information and lying to the families of Americans killed in the 2012 attacks in Benghazi, Libya.

Ben Carson, who has fallen behind his rivals despite being well-liked among Republican voters, generated laughs after joking about having to wait nearly 15 minutes to get his first question.

"I was happy to get a question this early on," the retired neurosurgeon said with a big smile.

Tighter rules for Thursday's debate, hosted by Fox Business Network, resulted in a smaller cast of candidates in the main event. Businesswoman Carly Fiorina was bumped to the undercard event, as was Kentucky Sen. Rand Paul, though he chose to not participate in the early evening contest.

Toward the end of the debate, a handful of audience members in the hall broke out into a "We want Rand" chant.

Republicans have one more debate scheduled — a Jan. 28 event in Des Moines — before voting begins in Iowa.